She started with 7. You're not given a starting value, but before she began cutting the pieces of paper, she grabbed 7 more. She ended with 28 half sheets. Basically, 7 × 2 = 14. Then, you multiply 14 by 2, which will get you 28 sheets. So she started with 7 sheets.
I hope this helps because I might sound complicated^^
I believe it's a linear function due to the fact you start with $70 and for every hour after, x, is $50 so there is no change in increase. So it is linear. Hope this helps!